Changes in the social behaviour of the desert locust, Schistocerca gregaria, in response to the gregarizing pheromone

Abstract

Experiments were carried out on second instar nymphs and immature adults of the desert locust, Schistocerca gregaria, to observe the effect of a pheromone, produced by crowded locusts, on the social behaviour of locusts reared in visual and tactile isolation. The three parameters of social behaviour measured (numbers touching, grouping, and at the arena edge) were not affected by the pheromone to the same extent or in the same direction. The pheromone facilitated the grouping behaviour of nymphs. Isolated nymphs themselves did not produce the pheromone; or if they did, it was in insufficient amounts to alter each others behaviour. The increase in grouping observed also in adults was not due to a direct effect of the pheromone on the adult stage, but was probably a residual effect from exposure during the nymphal stage. Exposure of isolated adults to the pheromone caused an altered distribution of touching locusts; compared with fully crowded and fully isolated adults, more were found in small groups (two locusts only) than in larger groups (three or more). The pheromone did not alter the rate of learning to group of nymphs or adults, either in the 2-hr test period or with further tests on adults. Isolated adults exposed to the pheromone showed a fall in grouping after some experience of social crowding. Choice experiments carried out on individual adults gave complex and inconsistent results.